Development of the Epidermis and Dermis
Early Development of the Epidermis
At four weeks, the embryo consists of:
Ectoderm (surface ectoderm): becomes the epidermis.
Underlying mesoderm (mesenchyme).
Initially, the cervicectoderm is a single cell layer thick.
After neurulation (spinal cord folding and closing) and embryo folding (around the fourth week):
Surface ectoderm proliferates.
Forms a single surface layer called the periderm (simple squamous epithelium).
Underlying layer becomes the basal layer (proliferating layer).
Periderm
Formation involves:
Keratinization: hardening and forming of the skin.
Desquamation: peeling away and shedding of cells into the amniotic fluid.
Cells detach and are released into amniotic fluid, often held together by a waxy substance forming the vernix caseosa.
Vernix Caseosa:
A thick, waxy, cheesy substance.
Protective layer for the skin of newborns.
Provides a moisturizing barrier between the delicate newborn skin and the external environment.
Important for the transition from the amniotic fluid to air exposure after birth.
There's variation among newborns; some have a lot, others have almost none.
Derived from the sloughing periderm layer.
Some of it is swallowed/ingested.
Role of the Periderm
The periderm's role is highlighted by lethal disorders where it is absent.
Cocoon Syndrome: A lethal disorder involving a nonsense mutation of I K K a.
Disrupts the protective barrier.
Associated with other birth defects.
Intermediate Layer and Epidermal Ridges
Epidermal ridges begin forming around ten weeks.
Form ridges on the inside, contributing to surface characteristics like fingerprints.
These ridges are down pullings of the intermediate layer into the mesoderm.
The pattern of ectoderm pushing into mesoderm is common during development.
Melanocytes (precursors: melanoblasts) migrate to the developing dermis and epidermis.
Immigrant cells:
Melanocytes (pigment-containing, neural crest-derived).
Langerhans cells (peripheral components of the immune system).
Formation of Epidermal Layers
Outer layer changes from periderm to stratum corneum (horny layer).
Beneath the stratum corneum lies the stratum lucidum (clear, thin layer of dead skin cells).
Other layers include:
Stratum granulosum.
Stratum spinosum.
Stratum germinativum.
Epidermal ridges form bulges pressing down into the mesoderm below.
Structural Development of the Epidermis
One-month embryo: ectoderm and mesenchyme.
Periderm forms as an outer layer and is sloughed off.
Two-and-a-half to three months: a basal layer forms with increasing mitotic division.
Fourth month: Outer layer sloughs off.
Sixth month: Periderm is mostly lost and becomes the cornified layer.
This outer layer hardens upon exposure to air.
Epidermal ridges form large troughs.
Skin Permeability
In utero, the fetal skin is typically permeable.
Near birth, the outer cornified layer becomes less penetrable.
This change coincides with:
Increased waste accumulation, especially urinary waste, in the amniotic fluid.
Reduces absorption of waste through the skin.
Clinical Conditions Related to Skin Pigment
Albinism: Congenital disorder with complete or partial absence of pigment in skin, hair, and eyes.
Associated with vision issues like photosensitivity and nystagmus.
Increased susceptibility to skin cancers.
Piebaldism: Affected cells can make pigment, but the function is turned off.
Localized.
Vitiligo: Chronic skin disease where portions of skin lose pigment.
Pigment cells die or become unable to function.
Possibly caused by autoimmune issues, oxidative stress, or viral causes.
Ichthyosis: Skin disorder causing dry, scaly skin.
Autosomal dominant inherited disease.
Mild cases may be mistaken for other skin conditions.
Severe cases can cause extreme itching due to the inability to sweat through thick skin scales.
Development of the Dermis
The dermis is mostly from the somatic layer of the lateral plate mesoderm.
Minor contribution from the somatic dermatome of the somite.
Dermis and developing mesoderm push in, creating dermal papillae.
Dermal papillae are projections of the dermis pushing into the epidermis.
Epidermal outgrowths and dermal papillae are an inductive process.
Formation of Fingernails
Fingernails form without a large inductive situation.
A thickening occurs at the tip of the fingers.
Thickenings sink down into the surface.
Form sunken proximal and lateral regions of the nail field.
Nail itself is formed by proliferation of cells of a toughened nail plate.
The epithelium goes forward over the nail bed.
Hair Formation
Humans have hair over almost their entire body.
Hair formation starts with epidermal downgrowths.
Dermal papilla (mesoderm) is involved.
Epidermal downgrowth plunges down and smushes the dermal papilla.
Hair papilla grows up and thrusts the bottom of the epidermis into a bump.
Hair papilla develops into a hair, which continues to grow and erupt through the surface.
The base of the hair is from the surface.
The hair itself grows over the dermal papilla (hair peg).
Sebaceous Glands
Develop on the side of the hair
Smooth Muscle
Almost all hairs are associated with a bundle of smooth muscle, derived from mesoderm.
Eyebrows and eyelashes are the exceptions.
Smooth muscles called arrector pili muscles.
Cause hair to stand on end when cold or scared (goosebumps).
Hypertrichosis (Werewolf Syndrome)
Abnormal amount of hair growth over the body.
Disruption in the rate of proliferation of hair.
Example of hair disorder with excessive hair follicles
Sweat Gland Formation
Similar to hair formation.
Bud from the ectoderm (epidermis).
Downgrowth of the epidermis forms sweat gland.
Downgrowth opens up to the surface.
Bottom coils, which forms secretory cells.
Smooth muscle is around the side walls of the coil.
Coils and develops a lumen that has an exit to the surface.
Sebaceous Gland Formation
Forms on the side of a hair.
Develops from the sides of the developing epidermal root sheaths.
Branches from the side and forms the ducts of the sebaceous glands.
Mammary Gland Formation
Starts with a thickening of ectoderm.
An entire ridge (mammary ridge), not just a single spot, forms.
A thickened ridge stretches from areola to groin on both sides.
Mammary Ridge:
Marks where can form mammary glands.
Programmed cell death occurs to remove nipple formation everywhere except where wanted.
Can form a nipple and or a breast anywhere along that line.
Accessory Nipples:
Accessory Nipple: when don't have cell signaling to regress and not form a breast.
Accessory nipples are super common (1 in 18 people).
Appear like a brown, raised mole.
Polyphelia: accessory nipple.
Polymastia: form an all out accessory breast.
Amastia: complete absence of breast, very rare.
Epidermis plunges down into the mesoderm but does not coil nor induce the formation of the dermal papilla like everything else does.
Modified sweat glands.
At puberty, the breast continues development primarily in females.
During pregnancy and lactation: marks the full development of the mammary gland.
Tooth Development
Derived from ectoderm and mesenchyme (or ectomesenchyme).
Enamel Organ: gives rise to ameloblasts, which produce enamel.
Dental Papilla (or Dental Follicle):
Dental Papilla gives rise to odontoblasts, which produce dentin and tooth pulp.
The follicle produces cementoblasts, which make cementum and osteoblasts that help make bone creating alveoli.
Fibroblasts: periodontal ligaments hold teeth in sockets, anchoring the roots into the jaw.
Tooth Development Stages:
Bud Stage (6-8 weeks): Epithelial cells proliferate into the ectomesenchyme of the jaw, which creates a dental bud.
Cap Stage: Dental papilla pushes up on the underside of the ectoderm; also look like formation of the hair root. Dental papilla pushes up on the underside of the ectoderm with the follicle growing around it.
Side note, Enamel Knot is formed during cap stage.
Bell Stage: Separates from the surface ectoderm and incorporates the dental papilla into the inner portion of the tooth.
Crown Stage: the tooth is hardening and calcifying. The hard tissues of enamel and the tissues on the outside are becoming fully formed.
Permanent teeth: are like the sebaceous gland of the hair cell which allows them which look/act like sidecars to eventually come up because the root starts do dissolve.